In many cases, metal members comprising iron as a main component are manufactured through working such as cutting and pressing. During manufacturing or when shipping as products, a rust preventive oil is applied for the purpose of preventing discoloration called rust or stain. The main purpose of the use of the rust preventive oil is to form a film on the metal surface to block oxygen, and thereby preventing oxidation of metal, that is, rust.
However, if a chloride or water is attached to the metal surface, it becomes difficult for the rust preventive oil to sufficiently exert its effect. For example, in the case where a metal working oil containing a chlorine extreme-pressure agent is used for a preceding process of working, in order to obtain a sufficient rust preventive property, the chloride that is a rust generation factor needs to be removed in advance. Thus, in many cases, after washing and removing the chloride with a finger print remover type rust preventive oil called NP-0 specified by JIS K 2246, a rust preventive oil is used.
Furthermore, when a water-soluble working oil is used, moisture is considered to be attached. Also in this case, moisture needs to be removed before using a rust preventive oil as is the case in chlorine. An oil solution whose water displacement property is enhanced is sometimes used to remove water. The water displacement property herein means performance by which an additive agent in the oil solution penetrates between water and metal, and water is removed from the metal surface. Oil solutions corresponding to NP-3-1 and NP-3-2 classified by JIS K 2246 also have a water displacement property, but this relates to a rust preventive property when moisture is mixed in a rust preventive oil and does not specify performance capable of removing water from a metal surface (see Non Patent Literature 1). Hereinafter, in order to avoid confusion, performance to remove water from a metal surface is called a “water removal property”, and is distinguished from the water displacement property specified by JIS.
As described above, in order to carry out rust preventive treatment for metal to which moisture is attached, firstly, moisture is removed by washing with an oil solution which excels in water removal property, and furthermore, a rust preventive oil needs to be applied. However, in many cases, the two processes, water removal and then rust prevention, cannot be carried out due to causes such as productivity and cost, and an oil solution capable of unifying treatment is required. However, it is very much a situation that an oil solution having a water removal property lacks a rust preventive property and a sufficient rust preventive property cannot be obtained by applying a rust preventive oil without a water removal process (for example, see Patent Literatures 1 and 2).
In order to unify processes, it is necessary to impart a sufficient rust preventive property (rust preventive performance) to an oil solution (washing agent) having water removal performance. However, a low-viscosity oil solution is advantageous for rapid water removal, whereas an applied oil film needs to have high viscosity for a sufficient rust preventive property. When viscosity is made to be intermediate so as to achieve both of the two kinds of performance, either performance may be insufficient.